1. Field of the Invention
The present invention relates to an ink jet head which prints images by ejecting ink droplets from ink ejecting nozzles, and in particular, relates to an ink jet head having a damper membrane which absorbs retrograde pressure wave transmitted from an ink cavity to an ink manifold and both vibration mode and a natural frequency of which are prepared so that ink ejecting performance is not affected by reflection pressure wave due to the retrograde pressure wave.
And the present invention relates to an ink jet head in which printing can be uniformly conducted while avoiding that the ink ejecting performance is affected by the reflection pressure wave, by giving absorbing function of the pressure wave occurring in the ink manifold to a flexible plate which is arranged between the ink cavity and an energy element.
2. Description of Related Art
Conventionally, as shown in FIG. 5, an ink jet head is basically constructed by combining a cavity plate 71 and a piezoelectric plate 72 while arranging a vibration plate 73 therebetween, and the ink jet head is installed on a base plate 74 which acts as a mounting base. To one side of the cavity plate 71, a nozzle plate 78 in which nozzle holes 75 (in FIG. 5, one nozzle hole 75 is shown) are formed is fixed. And a cavity room 76 for supplying ink to the nozzle hole 75 from the inside is formed in the cavity plate 71.
In the above ink jet head, a plurality of the cavity rooms 76 and the nozzle holes 75 are parallel arranged so that they form a multi-channel construction. Each cavity room 76 opposes to the piezoelectric plate 72 with the vibration plate 73 therebetween and when a vibration part 72a in the piezoelectric plate 72 is vibrated, the cavity room 76 is selectively pressed by the vibration of the vibration part 72a. In the cavity plate 71, an ink manifold 77 which acts as a common ink supply path for each cavity room 76 is formed.
In case that the piezoelectric plate 72 is driven and the vibration part 72a is vibrated, each cavity room 76 is selectively pressed and the pressure is transmitted to the corresponding nozzle hole 75, thereby ink droplet is ejected from the nozzle hole 75 and printing of images is conducted. At that time, the pressure wave occurring due to press of the cavity room 76 includes not only advance component directing toward the nozzle hole 75 but also retrograde component directing toward the ink manifold 77.
The retrograde component of the pressure wave is reflected in the ink manifold 77 and moves toward the nozzle hole 75 behind the advance component. Here, the reflection wave due to the retrograde component is dispersed in the ink manifold 77 because the manifold 77 is used as the common ink supply path for all cavity rooms 76, thus ink is not ejected from the nozzle hole 75 by only the reflection wave. However, the reflection wave affects recovery speed of ink quantity corresponding to ink quantity which is reduced by ejecting through the advance component of the pressure wave, therefore ink ejecting quantity and ink ejecting velocity are influenced at the next ejecting time. Further, since the extent of influence depends on how many channels are driven at the same time, that is, the extent of influence changes according to the number of the channels which are simultaneously driven, ink ejecting quantity and ink ejecting velocity fluctuates every ejecting time. As a result, it concludes that printing quality goes down.
In order to dissolve the above problem, it is disclosed in Japanese Patent Application laid-open No. Hei 3-30502, a drop-on-demand type print head in which a part of wall in the ink manifold (such part of wall is shown in FIG. 5 by an arrow A) is constructed from a flexible and soft film as a pressure damper. In the print head, the retrograde component of the pressure wave is absorbed by the flexible and soft film based on that volume change occurs in the ink manifold due to vibration of the film, thus formation of the reflection wave can be avoided.
However, even if a part of the wall in the ink manifold is constructed from the flexible and soft film as in the above print head, formation of the reflection wave cannot be completely avoided. Especially, there will occur a case that the flexible film vibrates under complexed mode where node(s) is/are formed on a plane of the film according to the shape or material of the film. Under such vibration mode the above volume changing effect of the ink manifold by the film is remarkably reduced, and thus the reflection wave with a big scale which cannot be neglected will occur. Further, the reflection wave moves toward the nozzle hole through the cavity room from the ink manifold, thereafter returns to the ink manifold after reflected again by the nozzle hole. Therefore, there will occur a case that resonance is produced according to the natural frequency of the film, as a result, damping of the reflection wave is remarkably delayed.
In the above cases, the reflection wave with a big scale which is not neglectable influences the ink supply property to the ink cavity from the ink manifold. Thus, ink ejecting quantity and ink ejecting velocity fluctuates based on how many channels are driven at the same time and it concludes that printing quality goes down.